1. Field of the Invention
The invention relates to pre-expanded plastic beads with a high dimensional thermal stability and a low bulk density based on poly-para-methylstyrene, as well as a process for the preparation of plastic beads containing a blowing agent to permit the preparation of said pre-expanded plastic beads.
2. Prior Art
The thermal stability limit of expanded polystyrene is 105.degree. C. If such cellular materials are expanded at higher temperatures, they warp and bend. For this reason, cellular plastics have been prepared from styrene/maleic acid anhydride copolymers, which are known to have a higher thermal stability than does polystyrene. If the known technology used for the preparation of expandable polystyrene (EPS) beads is applied to the styrene/maleic acid anhydride copolymers, one finds that the corresponding expandable plastic beads exhibit a relatively low expandability when processed on conventional EPS expansion equipment; bulk densities under 35 g/l are difficult to attain, and the rates of expansion are not satisfactory.
It is known that poly-para-methylstyrene also has a higher thermal stability than polystyrene. U.S. Pat. No. 4,237,255 describes the preparation of poly-para-methylstyrene in bead form through polymerization in an aqueous suspension. U.S. Pat. No. 4,242,465 relates to the preparation of foams based on poly-para-methylstyrene through the incorporation of a blowing agent in the molten polymer and extrusion to form continuous foam sheets.
If one also attempts to apply EPS technology here, one also obtains beads with insufficient expandability. An increase in the blowing agent content, for example, from 7 percent by weight, which is standard with EPS, to 9 or 10 percent by weight, does not produce a significant reduction in bulk density; in fact, once the pre-expanded beads are fused together, shaped objects are obtained with an irregular cellular structure which tend to shrink and collapse. The expandability can be improved by the addition of conventional plasticizers, such as cumene or ethylbenzene in the preparation of poly-para-methylstyrene containing blowing agents, but this is done at the cost of lowering the thermal stability of the foams produced therefrom to temperatures under 100.degree. C.
Thus, the purpose of the invention was to prepare pre-expanded plastic beads with an increased thermal stability which expand to lower bulk densities when they are prepared on conventional EPS processing machines.